Heat developable diazo compositions and diazotype reproduction media



3,529,964 HEAT DEVELOIABLE DIAZO COMPOSITIONS AND DIAZOTYPE REPRODUCTION MEDIA John F. Kienast, River Edge, and Edward O. Aicllelman, Irvington, N1, assignors to L. L. Ridgway Enterprises, Inc., Houston, Tex., a corporation of Texas No Drawing. Filed Jan. 14, 1966, Ser. No. 520,664- Int. Cl. GtlSc 1/60 U.S. Cl. 96-91 6 Claims ABSTRACT OF THE DISCLOSURE Diazotype reproduction media consisting of a supported light-sensitive heat developable diazo composition comprising 10-40 parts by weight each of a diazo compound and coupler therefor, at least 100 parts by weight by thiourea, at least 70 parts by weight of a noncolorific acid-to-neutral metal salt of a heat-responsive organic acid, and an acid stabilizer establishing a pH between about 2.5 and about 4 in said composition.

The present invention relates to heat developable diazo compositions and to media for producing diazotypes.

It has heretofore been proposed in the art to employ heat developable diazo compositions in reproduction media for producing diazotypes. Such heat developable media are viewed as highly desirable alternatives to the present semidry and dry diazo reproduction processes, both of which entail considerable disadvantages.

In the so-called semidry process, a photosensitive medium having a diazo compound in the light sensitive layer is exposed to light and then developed by contact with an alkaline developing solution containing a coupler. Coupling and image formation occur on contact of the exposed medium with the liquid developing agent. The semidry process has the disadvantage that relatively unstable, corrosive, alkaline developing solutions must be employed. The developed copies must be dried before use, and the prints tend to show background discoloration with age. In the dry process, a diazo compound, a coupler, and stabilizers are combined in an acid coating to produce a photosensitive imaging medium. After exposure, the pH of the exposed medium is raised by contact with ammonia fumes for development. The principal drawback of the dry process is the generation of the ammonia fumes used for development. Installations of this type must be adequately ventilated, or provided with special means for removing the ammonia fumes. In consequence, they are the for large part impractical for small scale oflice copying use.

Numerous heat-developable diazotype compositions have been suggested as alternatives to those used in the dry and semidry processes. In all of these compositions, a diazo compound and a coupler are combined in an acid environment together with a base release agent which will produce an alkaline environment in the medium upon heating. Diazotype reproductions are prepared using such media by exposing the media in the usual way to actinic light which decomposes and deactivates the diazo compound, and then heating the print at an elevated temperature to affect the base release agent and to create alkaline conditions for coupling of the diazo compound and cou pler, whereupon image formation appears in areas unexposed to light to give a positive reproduction of the image being reproduced.

The only heat developable diazotype media now commercially available take elaborate precautions to inhibit precoupling and to assure a satisfactory shelf life. For example, certain media presently commercially available employ thermally sensitive barrier layers to separate the 3,529,964 Patented Sept. 22, 1970 ice components of the system. On exposure to heat, the barrier layers break down to permit a diazo compound and coupler to react in an alkaline medium. Other commerical media encapsulate alkali in microcapsules having thermally sensitive shell walls. On exposure to heat, the shell wall material decomposes to release alkali into an acid-stabilized diazo-coupler combination, whereupon development and image formation occur. Still other heatdevelopable systems employ backside coating, in which the support or carrier sheet, usually paper, is used as a barrier to separate components of the diazo system. Upon heating, diifusion through the support layer occurs, whereupon a diazo compound and coupler in a photosensitive layer on the sheet are alkalinized, couple, and form an image.

Heat-developable systems which are considerably simpler than those described above have been proposed in the art. For example, diazotype media comprising a diazo compound, a coupler, acid stabilizing agents, and urea and/ or an alkyl urea as the heatsensitive base release material have been described in the patent literature. On heating, the urea or substituted urea decomposes with the release of ammonia which alkalinizes the diazo composition, effecting coupling of the diazo compound and coupler.

Heat-developable systems comprising ureas have not been accepted in the market, despite their obvious simplicity when compared with the commercial heat developable systems described earlier herein. The ureas proposed as base release agents in these systems are relatively readily decomposed and produce other fumes in addition to ammonia which are corrosive and damaging to mechanical equipment used to develop diazotype prints. These corrosive fumes eventually attack the heated metal rollers and other parts present in copy machines used for the thermal development of diazotypes. A further disadvantage of diazo composiitons containing ureas is the slow reaction of such ureas with the acidic stabilizing substances present in the compositions. This: reaction results in a gradual attrition of the photosensiitve compositions and detracts from the shelf life of diazotype reproduction media made therefrom.

According to the present invention, lightand heatsensitive compositions have been discovered which are comparable to their print speed, development speed, and shelf life with the papers employed in the ammonia-developed dry process now most popular commercially. The developed prints are of exceedingly good quality, with very low background and an exceptional print stability. As compared with the other heat developable compositions and diazotype media now commercially available the systems of the present invention are far less expensive to formulate and far easier to prepare, because complex techniques such as encapsulation, barrier layer formation, etc., can be avoided entirely. Also, for the same development temperature and time, the heat developable media of the present invention give denser images than are obtainable with the commercial heat developable materials. Finally, as compared with the heat developable systems proposed in the art and employing ureas as the base release agent, the compositions of the invention do not generate corrosive fumes, even when employed at the highest temperatures feasible for heat development.

The diazo compositions of the present invention comprise conventional diazo compounds and azo couplers in combination with thiourea, certain metal salts of volatile and/or heat-decomposable acids, and free acid stabilizers establishing a pH in the compositions between about 2.5 and about 4.

A large number of diazo compounds and couplers are known in the art, and no criticality has been found in the choice of particular diazo compounds or coupling components for use in the compositions of the present invention. A wide variety of typical diazo compounds and couplers is disclosed and discussed in detail on pages 194, et seq., of Light Sensitive Systems, by Jaromir Kosar, John Wiley & Sons, Inc., New York, 1965.

Certain diazo and coupler combinations are preferred from the viewpoint of stability and activity. For example, a diazo compound which has proved particularly useful in formulating the composition of the present invention is the full zinc chloride salt of p-diethylaminobenzene diazonium chloride. The half-zinc chloride salt of p-diethylaminobenzene diazonium chloride may be employed, as may p-diethylaminobenzene diazonium fiuoroborate, for example. Other suitable diazo compounds include p-diethylamino 2 ethoxybenzene diazonium chlorozincate, 4-diethylamino 2 methylbenzene diazonium chlorozincate, p-dimethylaminobenzene diazonium chlorozincate, p-amino N ethyl N hydroxyethylbenzene diazonium chlorozincate, p-morpholinobenzene diazonium fluoroborate, 4 morpholino 2,5-diethoxybenzene diazonium chlorozincate, etc.

A preferred coupler for use in the compositions of the present invention is the sodium salt of 2,3-dihydroxy-6- naphthalene sulfonic acid. Other couplers, however, include materials such as 2,3-dihydroxy naphthalene, 2,7- dihydroxy-3,6-naphthalene disulfonate, and acetoacetanilide. For the formation of sepia and black prints, resorcinol and various diphenyl derivatives are particularly suitable alone or in combination 'withother couplers. The diphenyl derivatives include 2,2',4,4' tetrahydroxydiphenyl and the corresponding 2,2'4,4' tetrahydroxydiphenylsulfide, and 3,3',5,5' tetrahydroxydiphenyl and 3,3',5,5' tetrahydroxydiphenylsulfide. As mentioned earlier, a wide variety of couplers and their properties are discussed and tabulated in the Kosar reference work, and the reader is directed to that reference for further examples of materials adaptable to use in the present invention.

Thiourea has long been employed in the art as an additive to diazo compositions to stabilize prints. This sta bilizing effect is generally attributed to the fact that thiourea is a reducing agent and counteracts darkening of print background by oxidative processes. Although the diazo compositions of the present invention also contain thiourea, and although the exceptional print stability observed in diazotypes made using these compositions may be attributable to the thiourea content of the compositions, the compositions of the present invention should not be confused with those of the prior art containing thiourea only as a minor stabilizing additive. In the diazo compositions of the present invention, thiourea is present in large amounts as a principal and critical component and appears to act as the base release agent which, on heating, causes alkalinization of the compositions to permit coupling.

The thiourea is used in combination with certain metal salts, defined herein as non-colorific acid-to-neutral metal salts of heat responsive organic acids. Non-colorific salts are salts which are colorless or substantially colorless (i.e., salts which do not per se contribute objectionable color to diazo compositions in which they are contained), and which do not react with components of the composition (particularly the couplers) to form colored products. Acid-to-neutral salts are those whose solutions in water have a substantially neutral or slightly acid pH, i.e., about to about 7. The salts of heat-responsive organic acids are salts of volatile and/or heat-decomposable acids, suitably salts of those acids boiling and/ or decomposing below about 170 C.

Salts of formic, acetic, propionic, lactic, malic, maleic, and tartaric acids are salts of typical heat-responsive acids, as contemplated herein. The zinc, magnesium, calcium, and barium salts, for example, of such acids are suitably acid-to-neutral and noncolorific. In contrast, the corresponding sodium or potassium salts give alkaline aqueous solutions by hydrolysis (i.e., are not acid-toneutral, as contemplated herein), and the salts of metal ions such as iron, nickel, cobalt, manganese, copper or chromium are objectionably colored or react with typical couplers to form colored compounds, as is known to those skilled in the diazo arts.

In the compositions according to the invention, the metal salts are present in amounts ranging from 10 parts by weight to the limits of solubility in the aqueous solutions used in depositing coatings of the compositions on a substrate. The role of the metal salts in the compositions of the invention is not known in detail. For example, to what extent the salts may participate in producing alkaline conditions in the coating on heating is not known. One striking function of the salts, however, is their solubilization of thiourea in preparing aqueous solutions of the diazo composition for use in coating. The normal solubility of thiourea in water at 25 C. is less than about grams per liter. However, in preparing the composi tions of the present invention in which salts such as zinc acetate are present in combination with thiourea, thiourea can be brought into solution in amounts as high as 300 grams per liter.

The pH of the diazo compositions of the invention is between about 2.5 and 4, preferably between about 3 and 4. An optimum value for the pH is about 3.5, particularly for those compositions employed to give a blue line image. The pH may be somewhat lower for the production of black line images, which are conventionally produced by combining a diazo compound with couplers which, individually, give blue and yellow or sepia prints. At a pH above 4, the compositions may be subject to precoupling at temperatures below the temperatures used for heat development. At a pH below about 2.5, development at feasible temperatures is no longer possible.

The acids used to adjust the pH of the compositions within the limits noted may be organic or inorganic, and may be weak acids or strong acids. However, weak organic acids having a dissociation constant of about 10 are preferred. Organic acids such as citric, formic, tartaric, lactic, acetic, malic, benzoic, and maleic acids are exemplary of those which can be employed. Inorganic acids such as hydrochloric acid or phosphoric acid may also be employed. However, since the weak organic acids permit the production of the desired pH without the necessity for corrosive solutions, they are preferred.

The diazo compositions of the invention may optionally contain additional stabilizing agents such as caffeine or zinc chloride. The former is known in the diazo arts as particularly useful for solubilizing difliculty soluble couplers.

The diazo compositions of the present invention can be used for the sensitization of suitable porous or nonorous supports or carriers. However, they are of greatest utility when used with a fibrous web support such as of cloth or paper. Papers which are standard for direct process coating are widely available commercially, and relatively iron-free papers particularly suitable for use as supports for diazo compositions are known in the art and are readily purchased.

The photosensitive and heat-sensitive compositions of the present invention can be directly coated on a substrate such as a paper to give excellent prints. It is preferred, however, to precoat papers used in preparing diazotype media to decrease their porosity and to increase their effective surface area prior to coating with a diazo composition. For this purpose, finely divided materials such as silica and other inorganic pigments of high surface area are often combined with a resin acting as a binder and sealant. The resin prevents the subsequently applied diazo composition from sinking deeply into the paper, where it would be diluted with cellulose fibers, but retains it on the surface for maximum image visibility and the development of dense color. The finely divided particles used in the precoat increase surface area, print density, and vividness of the image.

A number of precoating techniques are disclosed on pages 296, et seq., of the aforementioned Kosar reference. Particularly good success has been had with the diazo compositions of the invention using a precoat in which colloidal silica, starch, and barium sulfate are combined With a binder such as polyvinyl alcohol or polyvinyl acetate. The particulate fillers improve the smoothness and whiteness of the background, and conceal the fibrous structure of the paper support so that pencil lines and the like appear smoothly continuous in the diazotypes.

When a precoat is applied, it is applied in amounts of approximately 24 grams per square yard. Since the use of any precoat is optional, less dense applications are also acceptable. Similarly, the amount of precoat may 'be increased, but presents no positive advantages while entailing an economic disadvantage. The photosensitive coats of the present invention are generally applied to a substrate in amounts between 4 grams per square yard and 8 grams per square yard.

Both the precoat and the sensitive coat can be applied with conventional commercial paper coating equipment. Particularly good results have been employed by applying either the precoat or the coat to the paper with a roller and then removing excess with a blade or air doctor. If desirable to minimize curling, the papers may also be back-coated.

The diazotype reproduction media of the present invention can be exposed and developed in reproduction apparatus now commercially available for heat development processes. As for other diazo compositions, the media of the invention are briefly exposed to a light source, usually an ultraviolet or near-ultraviolet source. In illuminated areas, the diazo compound decomposes with release of nitrogen to form noncoupling products. The exposed print is then developed by heating to a temperature between about 275 F. (130 C.) to about 350 F. (about 175 C.). Development is slow at temperatures below the minimum temperature indicated. While the compositions of the invention will form color at temperatures above the maximum temperature indicated, higher temperatures may be detrimental to a paper support, or to the flexible belts used to transport the reproduction medium through conventional development apparatus. In general, temperatures of about 300 F. (150 C.) are optimum.

It is surprising, if thiourea is functioning as a base release agent in the compositions of the invention, that the compositions can be heat developed at temperatures as low as those indicated. Thiourea is converted by heat in ammonium thiocyanate, its isomer, to form eutectic mixtures. Decomposition to form ammonia is indicated in the literature to take place at about 200 C., a temperature considerably above those at which the diazo compositions of the invention can be developed.

Development time varies with development temperature and with the speed and sensitivity of the diazo compound and coupler, as is well known in the art. In general, shorter development times are required at higher temperatures than at lower temperatures, and vice versa.

In contrast with some commercially available heat developable diazo systems, the developed prints of the present invention produce waterproof images which do not bleed or dissolve when exposed to water.

In formulating the sensitizing solutions of the present invention prior to coating, there is no criticality in the order of combining ingredients providing that an acid pH is maintained to inhibit precoupling of the diazo compound and coupler. The metal salts may be added as salts, or may be formed in situ by a combination of metal oxides, hydroxides, carbonates, etc., with appropriate acids.

A better understanding of the present invention and of its many advantages will be had by referring to the following specific examples, given by way of illustration.

6 EXAMPLE 1 A diazo composition suitable for the preparation of blue line prints was prepared by mixing the following ingredients in one liter of water.

5 Component: Amount in grams Zinc acetate 100 2,3-dihydroxy-6-3naphthalene sufonic acid, so-

dium salt 24 l 0 p-Diethylaminobenzene diazom'um chloride -ZnCl 26 Thiourea 160 Citric acid 3O Lactic acid 36 Zinc chloride The composition, which has a pH between about 3.5 and about 3.7, was used to sensitize both paper sheets without a precoat and paper sheets precoated with a mixture of finely divided silica, barium sulfate, starch, and a polyvinyl alcohol binder.

The sensitized sheets were exposed through an original to a mercury vapor lamp source for about 11 seconds, and then were contacted with a heated roller at about 300 F. for about 11 seconds.

In both cases, brilliant blue positive images were produced, the images on the precoated sheets being denser and more attractive.

Equivalent results were obtained by substituting either barium formate or calcium acetate for the zinc salt. The amount of salt can be reduced considerably below the value shown in the table, or increased to saturation levels, e.g., about 300 grams/liter for zinc acetate.

Also, the lactic and citric acids present can be replaced by one or more other acids, as described, to establish a pH as indicated. Caffeine may optionally be included in the composition and speeds solution of the coupler in the coating solution and promotes stability. The zinc chloride present is also optional. When present it plays 40 a stabilizing role and also contributes in part to establishing acidity in the solutions.

In general, aqueous coating solutions of the type shown in these examples may be made up containing between about 10 to 40 grams each of the coupler and the diazo component per liter of water. A concentration of more than about 40 grams per liter of coupler tends to give coating solutions which are unstable. The amount of diazo compound employed with a given amount of coupler is variable depending on the speed and density desiredin the composition, as well known to those skilled in the art.

Thus, the specific preferred formula given earlier can be generalized as follows:

Component:

Zinc acetate, (or

other metal salts).

Amount in grams -300 (satn) (in equivalent amounts).

By employing a different coupler, the same diazonium salt as in Example 1 can be used to give sepia prints. For example, se'pia prints were prepared on papers with and without a precoat by applying a sensitizing layer of the composition shown specifically below dissolved in one liter of water. (More general composition limits are also tabulated).

Amount in grams On exposure and development by contact with a heated roller, good sepia prints were produced.

EXAMPLE 3 By employing a mixture of the couplers used in Examples 1 and 2, the same diazonium salt can be used to prepare sensitizing solutions suitable for the production of black line prints. For such a solution, the ingredients shown below were dissolved in one liter of water. (More general composition limits are also given.)

Amount in grams Component Specific Zine acetate 128 70 300 30 3G 40 40 2,3-dihydroxy-6 naphthalene sulfonic acid, sodlum salt 24 10-40 p-Diethylaminobenzene diazonium chloride ZllClz. 26 1040 Thiourea 100 100-300 3,3 ,5,5-tetrahyd1'oxy diphenylsulfide 12 5*20 2 To give an equivalent pH.

The solution had a pH between 3.2 and 3.6 and, as in the earlier examples, was used to treat paper sheets with and without a precoat.

On exposure and heat development, good black images were obtained.

EXAMPLE 4 Another composition giving suitable blue line prints was prepared by dissolving the following ingredients in one liter of water.

Component: Amount in grams Magnesium formate 75 Citric acid 30 Lactic acid 72 (30 cc.) 2,3-dihydroxy-6-naphthalene sulfonic acid, so-

dium salt 24 p-Dimethylaminobenzene diazonium chloride-ZnCl 26 Thiourea 160 The composition was applied to sheets with and without a precoat, as in the other examples.

What is claimed is:

1. An image reproduction medium for the production of positive diazotypes, said medium comprising a support and a light-sensitive heat-developable diazo composition thereon, said composition consisting essentially of 1040 parts by weight of a light-sensitive diazonium compound, 1040 parts by weight of an azo coupler, at least parts by weight of thiourea, at least 70 parts by weight of a noncolorific acid-to-neutral salt formed between a metal selected from the group consisting of zinc, magnesium, calcium, or barium, and a heat-responsive organic acid selected from the group consisting of formic, acetic, propionic, lactic, malic, maleic, or tartaric acid, and and acid stabilizer establishing a pH between about 2.5 and about 4 in said composition, the maximum numbers of parts of thiourea and of said metal salt Permissible in said composition being those numbers of parts which saturate an aqueous solution of said heat-developable diazo composition in 1000 parts of water, said heatdevelopable diazo composition being deposited on said support from an aqueous solution in which all said essential ingredients are dissolved.

2. An image reproduction medium as in claim 1 wherein said acid stabilizer in said composition includes a weak organic acid.

3. An image reproduction medium as in claim 1 wherein said acid stabilizer is formic, acetic, citric, lactic, malic, maleic, or tartaric acid.

4. An image reproduction medium as in claim 1 wherein said coupler for said diazo compound includes 2,3-dihydroxy naphthalene, 2,3-dihydroXy-6-naphthalene sulfonate, resorcinol, 2,2',4,4-tetrahydroxydiphenyl, 2,2, 4,4-tetrahydroxydiphenylsulfide, 3,3,5,5-tetrahydroxydiphenyl, or 3,3,5,5'-tetrahydroxydiphenylsulfide.

5. An image reproduction medium as in claim 1 wherein said diazo compound includes the diazonium chlorozincate or diazonium fiuoroborate of p-diethylaminobenzene, p-dimethylaminobenzene, or 4-morpholino-2,5-diethoxybenzene.

6. An image reproduction medium as in claim 1 wherein at least parts by weight of thiourea are present.

References Cited UNITED STATES PATENTS 1,870,930 8/1932 Spongerts 96-91 2,653,091 9/1953 Greig 96-91 3,298,834 1/1967 Eldred et al. 96-91 3,326,686 6/1967 Randall et a1. 969l 3,360,371 12/1967 Munder et al. 96-91 3,367,776 2/1968 Bialczak et al. 96-91 3,389,996 6/1968 Welch 969l 3,431,109 3/1969 Bialczak et al 9649 FOREIGN PATENTS 907,724 10/1962 Great Britain.

909,491 10/ 1962 Great Britain.

983,665 2/1965 Great Britain. 1,301,867 7/1962 France.

NORMAN G. TORCHIN, Primary Examiner C. L. BOWERS, Assistant Examiner US. Cl. X.R. 96-49 

